In a typical chain transmission in which a roller chain or a bushing chain engages a sprocket, the sprocket is defined by an industrial standard, such as the Japanese Industrial Standard (JIS). The Japanese Industrial Standard JIS B1801-1997 and its attached “paper 2” also define sprocket tooth forms, including S-tooth forms, U-tooth forms, and ISO-tooth forms).
The standard describes the following expression.pa=p(1+(ds−d1)/d)in which,                pa is the tooth form pitch;        p is the chain pitch;        ds is two times the radius of the arc of the tooth gap bottom;        d1 is the outer diameter of a roller; and        d IS the diameter of the pitch circle.In this specification, the terms “standard roller chain” and “standard bushing chain” refer to roller chains and bushing chains as defined in JIS B1801-1997, and the term “standard sprocket” refers to a sprocket having S-tooth form or a U-tooth form, as defined in its attached paper 2.        
A standard roller chain comprises inner links each composed of a pair of opposed inner link plates connected by two bushings, the ends of the bushings being press-fit into bushing holes in the link plates, and a roller rotatable on each bushing. The chain also comprises outer links, each composed of a pair of opposed outer link plates connected by a pair of pins press-fit into pin holes in the outer link plates. The links are interconnected by having a pin of each outer link extend through a bushing of an inner link. In this way, the chain is formed of alternating inner and outer links, with the links connected to one another flexibly so that the chain can bend around a sprocket. The standard roller chain has a uniform chain pitch p, which corresponds to the distance between the centers of adjacent rollers.
The standard sprocket is shown in FIG. 4. In the standard sprocket, tooth surfaces, which are continuous with an arcuate tooth gap bottom, are symmetrical with respect to center lines x which extend radially from a center of rotation O through the centers of the tooth gap bottoms. The center lines x intersect the pitch circle pc at intersection points a, and the angle between adjacent lines x is the tooth pitch angle θ. The tooth pitch angle θ is related to the number z of teeth on the sprocket, by θ=360°/z.
The tooth form pitch pa is a distance between adjacent intersections a. That is, the tooth form pitch pa is the length of a chord corresponding to the tooth pitch angle θ. Since the standard sprocket has uniform tooth pitch angles θ, the tooth form pitch distances pa along the circumference of the pitch circle pc are equal. Furthermore, the tooth form pitch pa has a fixed relation to the chain pitch p as expressed by the formulapa=p (1+(ds−d1) /d).
U.S. Patent Publication 2005/0119079 describes a roller chain transmission, which reduces noise generated when a roller of a roller chain engages with a sprocket. The roller chain transmission comprising a roller chain, and a sprocket which has at least first teeth and second teeth having different tooth forms. The first teeth and second teeth are randomly distributed on the periphery of the sprocket, the number of second teeth does not exceed four times the number of first teeth, and the teeth immediately preceding and following each of the first teeth on the periphery of the sprocket are second teeth.
In FIG. 4, the rollers of a standard roller chain 1 are shown engaging a standard sprocket 3 in a conventional chain transmission. The standard roller chain 1 has a uniform chain pitch p (the distance between the centers O1 of the rollers 2). The standard sprocket 3 is a driving sprocket having eighteen teeth. Since the tooth pitch angle θ is determined by the expression θ=360°/z, the tooth pitch angle θ of the driving sprocket is 20°. Furthermore, as described above, the tooth form pitch pa is the length of a chord corresponding to the tooth pitch angle θ. Therefore, the standard sprocket 3 has uniform tooth pitch angles θ of 20° and a uniform tooth form pitch pa.
When the standard sprocket 3 is rotated clockwise, at the beginning of engagement of a roller 2, the roller is moved relative to the center O1 of a preceding roller that has already become seated on a tooth gap bottom, in an arc having a radius corresponding to the chain pitch p. The roller then collides with the center of a tooth gap bottom substantially at a right angle. The kinetic energy of the roller 2 is transmitted to the tooth gap bottom portion without being buffered. The collision of the roller with the tooth gap bottom results in significant vibration and noise.
Further, since the tooth form pitch pa has a fixed relationship with the chain pitch p according to the expression pa=p(1+(ds−dl)/d), each roller 2 abuts a tooth gap bottom of the standard sprocket 3 at the same position t, as shown by a circle marks in FIG. 4. As a result, the collisions occur at a particular frequency depending on the rotational speed of the sprocket and the noise and vibration have a corresponding frequency.
FIG. 5 is a graph showing a result of noise measurement on the conventional chain transmission of FIG. 4. The noise measurements were carried out with a transmission having a standard sprocket with eighteen teeth as a driving sprocket and a standard sprocket having thirty six teeth as a driven sprocket. The rotational speed of the driving sprocket was varied over a range from 500 to 5,000 rpm.
According to FIG. 5, overall sound designated “OA” represents the entire noise generated by the transmission. The OA level in FIG. 5 is close to the level of each rotational order sound. That is, the area S1 between the level of each rotational order sound and the level of the overall sound OA is small. Thus, in the conventional chain transmission, the rotational order sounds are conspicuous and grating to the ear.
The approach described in U.S. Patent Publication 2005/0119079 results in a noise reduction, but, because the sprocket has at least two different tooth forms, its manufacture is complex and its manufacturing cost is high.
Accordingly, an object of this invention is to provide a chain transmission in which vibration and noise generated when a roller of a roller chain or a bushing of a bushing chain engage a sprocket are reduced, and in which the manufacturing cost of the sprocket is reduced.